4 data flow – Guralp Systems CMG-6TD User Manual
Page 63
Configuration with Scream!
The Duration (minutes) box tells the digitiser how long to provide the
calibration signal before disconnecting. This avoids the system being
inadvertently left in calibration mode. The default is two minutes. If you
change this setting, it will revert to the default value after one calibration
stage.
Three calibration methods are available: Sine Wave, Square Wave (step) and
Broadband Noise.
The Sine Wave calibration signal always starts and stops on the zero crossing.
The frequency or period are specified by the boxes at bottom left. Only
integers between 1 and 10 may be specified for either frequency or period, so
to generate a 0.5 Hertz signal you should select Period and set the time to 2
(seconds). Likewise, if you require a 0.25 second period you should select
Frequency and set the rate to 4 (Hertz). In this manner, you can select
frequencies ranging from 0.1 to 10 Hertz (10 to 0.1 second periods).
You can specify step calibration by selecting the Square Wave (step) button.
The square wave consists of a positive step at the start of the next minute of
the digitiser’s internal clock, followed by a negative step after a specified
number of minutes. After a further delay of the same number of minutes, the
calibration signal is disconnected. The default is two minutes. The Period
and Frequency are ignored.
The Broadband Noise calibration signal consists of a constant stream of
pseudo-random noise, which lasts for the specified number of minutes. The
Period and Frequency are ignored.
Broadband noise calibration, the most commonly used method, is described
in section 6.5.1 on page 75. Other calibration methods, including the
interpretation of the results, are fully described in the Scream manual,
MAN-SWA-0001.
5.2.4 Data flow
The 6TD operates in one of several transmission modes. These modes relate
to how the unit uses its Flash memory:
•
as a simple data store, from which you can request data (FILING, DUAL
and DUPLICATE modes);
•
as a buffer holding unacknowledged blocks, which are transmitted in
preference to real-time data (FIFO mode);
•
as a buffer holding unacknowledged blocks, which are transmitted
whenever the channel is free but no real-time data blocks are ready
(ADAPTIVE mode); or
•
not at all (DIRECT mode).
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Issue F - February 2014